I feel like a bit of a n00b because I find the Monoid docs not really helpful in understanding how t...

# arrow

[kierans777]

I feel like a bit of a n00b because I find the Monoid docs not really helpful in understanding how to use Monoids in Arrow (I use them regularly in other FP programs). I have a List<Map<String, String>> and I want to fold it down to Map<String, String> . How could I do this using a Monoid in Arrow?

[simon.vergauwen]

Hey @kierans777,

Iterable has an extension combineAll that takes an Monoid argument, and if you look at the Monoid.Companion you will find all instances.


On the Monoid.Companion we find Monoid.map()
https://arrow-kt.io/docs/apidocs/arrow-core/arrow.typeclasses/-monoid/-companion/index.html

[simon.vergauwen]

Which can be used together with combineAll which you can find on the Extensions page of Arrow Core.

https://arrow-kt.io/docs/apidocs/arrow-core/arrow.core/index.html#functions

https://arrow-kt.io/docs/apidocs/arrow-core/arrow.core/combine-all.html

[simon.vergauwen]

We've put a lot of effort into the docs, but as in many projects, it's the very hardest part. Any help or contribution is always very much appreciated if you have some ideas for improving the docs on Monoid.

I'm always happy to help if you have any questions in regards to contributing! So feel free to reach out here or in DM.

[kierans777]

Thanks for the reply @simon.vergauwen. Could you please give me an example of how to fold a List<Map<String, String>> to a`Map<String, String>` using a Monoid? I don't understand why Monoid.map requires an argument.

val x = listOf(mapOf("one", "1"), mapOf("two", "2"), mapOf("three", "3"))
val y = ???

// I want y to look like
y = {
  "one": "1"
  "two": "2"
  "three": "3"
}

[simon.vergauwen]

The reason for the argument to map is because you need to be able to combine values for duplicated keys.

val x = listOf(mapOf("one" to "1"), mapOf("two" to "2"), mapOf("three" to "3"), mapOf("three" to "3"))
val y = x.combineAll(Semigroup.map(Semigroup.string())) // mapOf("one" to "1", "two" to "2", "three" to "33")

[kierans777]

OK, in a statically typed language I can see why you need to do it like that.